One type of combined cycle gas turbine power plant utilizes at least one gas turbine and at least one steam turbine, in combination, to produce electric power. The power plant is arranged such that the gas turbine is thermally connected to the steam turbine through a heat recovery system such as a heat recovery steam generator (“HRSG”). The gas turbine generally includes a compressor section, a combustion section disposed downstream from the compressor section and a turbine section which is downstream from the combustion section. A rotor shaft of the gas turbine is coupled to a generator. A rotor shaft of the steam turbine may be coupled to the same generator or to a separate generator.
The HRSG generally includes one or more heat exchangers which are positioned downstream from a turbine exhaust duct of the gas turbine. During fired operation of the gas turbine, hot combustion exhaust gases flow from the exhaust duct, through the HRSG and out an exhaust stack. Thermal energy from the hot combustion exhaust gas is transferred via the heat exchanger(s) to a working fluid such as water so as to provide a flow of pressurized steam to the steam turbine(s).
In certain instances, the gas turbine may be operated primarily during peak or high power demand periods and shut down during non-peak or low demand periods. During a shut-down or non-fired operational period, however, it may be generally desirable to keep the rotor shaft of the gas turbine rotating at some desired minimal rotational speed via a turning gear which is coupled to an electric motor in order to protect the gas turbine rotor from bowing.
As the rotor is turned via the turning gear, ambient air is drawn through the compressor section, routed into a compressor discharge casing of the combustion section, routed through the turbine section out the exhaust duct and then through the HRSG. Although the air flowing from the compressor during turning gear operation may realize a slight increase in thermal energy, the temperature of the air passing from the compressor into the HRSG may be lower than the temperature of the working fluid residing in the heat exchangers of the HRSG, particularly soon after fired-operation of the gas turbine has ceased. As a result, thermal energy from the working fluid within the heat exchanger is lost to the cooler exhaust air.
The loss of thermal energy from the working fluid in the HRSG during turning gear operation may negatively impact overall power plant performance. For example, additional time may be required to bring the working fluid within the HRSG back to a required operating temperature before full operation of both the gas turbine and the steam turbine may be realized. In addition, a large temperature differential between the working fluid in the HRSG and the hot turbine exhaust gas, particularly during the initial start-up period, may result in thermal stresses on various components of the HRSG which may impact overall HRSG performance. Accordingly, a system and method for conserving thermal losses from the HRSG working fluid during turning gear operation of the gas turbine would be useful.